X-Y graphical input devices are relatively well-known in the computer arts for inputting graphical data such as handwritten text, symbols, drawings, etc., where it is desired to convert the instantaneous position of the stylus on a tablet into the X-Y coordinates which represent the absolute position of the stylus on the tablet surface. It is also frequently desirable to have a transparent X-Y digital input tablet which can be mounted directly on the surface of or over a flat screen display device such as are becoming widely available.
Currently available transparent digitizing tablets have not been found to be suitable for hand written text recognition in terms of resolution, accuracy, robustness in typical office environments, and cost. Inductive tablets operating at radio frequencies tend to have problems with ambient electrical noise from computer systems likely to be in the office area, display terminals, and other sources, as well as field distortions from the user's hand and/or nearby metallic objects such as hand jewelry. Tablets employing resistive surfaces for ranging are prone to manufacturing difficulties in obtaining uniform resistivity, and drift in the resistivity over time because of dirt, moisture, and aging. Acoustic tablets suffer distortions from hand contact on the surface, as do capacitive tablets. Laser scanning tablets are bulky, expensive, noisy, and have potentially unreliable mechanical parts. All of these tablets have active tablet surfaces that need to be attached to the computer system by a cable, and generally limit the use of the scribing instrument to a single tablet. The system of the present invention avoids all of these difficulties.
The prior art is replete with many different types of stylus-tablet input systems for entering data into a video display system in many different forms. Most of the systems capable of providing X-Y information to the video display utilize active tablets wherein the means for sensing the stylus position is built into the tablet per se and requires a physical connection between tablet and the system. Systems such as these merely utilize a passive stylus although some systems known in the art require both an active stylus and tablet.
The prior art in the area of inert or passive tablets fall into one of two classes. The first although not really a pen input system would merely use a non-specific tablet or surface and utilize some means of entering relative positional data into the system such as by means of the well known mouse inputs. However, such a system although it could possibly work, is not practical for the input of hand written data or the like.
The second class of tablets which would normally utilize a stylus type of pick up device again gives relative position of the stylus beginning at some predetermined origin on the tablet surface and as the stylus is passed over the surface physical pick up means are provided such as by grid lines to indicate relative movement of the stylus in the X or Y direction. Such systems are not capable of producing sufficiently dense data e.g., suffer from lack of resolution for such applications as for example, hand written or complicated and detailed graphical drawings.
There are many different types of digitizing tablet-stylus systems utilizing an active tablet wherein some physical or electrical parameter of the tablet is changed by the presence of the stylus on the surface of same. However these systems result in tablets which are fairly bulky and usually non-transparent. They also suffer from lack of resolution, low sampling rate and considerable inaccuracy or drift in the values produced and generally are unsuitable for such applications as handwritten text input and the like.